Modelling of aerosol modification resulting from passage through a hill cap cloud

Data collected at the Great Dun Fell site are used to provide input and comparative output for a computer model of the hill cap cloud system. The aim of the report is to investigate the effect of in-cloud chemical processing on aerosols and trace gases, focusing on the production of S(VI) via the oxidation of SO2 by H2O2. Both airflow, cloud chemistry and cloud microphysics are modelled and compared to upwind, downwind, and summit observations made during the Great Dun Fell field campaign in 1995. The results indicate that there exists a broader droplet size distribution than predicted due to the mixing of separate parcels of air with different trajectories and humidities. Modification of the aerosol size distribution is predominantly due to sulphate production at the expense of sulphur dioxide gas oxidised by hydrogen peroxide. Predicted nucleation scavenging resulted in the loss of the more hygroscopic particles of diameters 0.05–0.13 μm, which by the addition of soluble mass grew to between 0.13 and 0.3 μm in diameter. The less hygroscopic mode comprised approximately 2% of the total mass input and thus did not significantly contribute to the modified sections of the aerosol spectrum. The modified particles were of a size suitable for nucleation scavenging, increasing the number of CCN available for future droplet activation. The hygroscopic properties of the modified particles were also affected by the addition of soluble mass, such that they would require a lower critical supersaturation for activation (Swietlicki et al., 1999). The level of aerosol augmentation is dependent upon the activation history of the cloud droplets, the concentrations of interstitial gas species, and the partitioning of the aerosol ion species.